The invention relates to a process for the polymerization of at least one olefin in the presence of at least one catalyst comprising at least one Exe2x80x94Mxe2x80x94X linkage, in which E represents an oxygen or sulfur atom, M represents a nickel or palladium or platinum atom and X represents a phosphorus, arsenic or antimony atom, in a medium comprising a continuous liquid phase which comprises more than 30% by weight of water. The liquid phase comprising more than 30% by weight of water is subsequently referred to as the xe2x80x9caqueous phasexe2x80x9d.
The polymerization of olefins by Ziegler catalysis usually involves highly hydrolyzable, indeed even pyrophoric, compounds (catalyst and cocatalysts) and it is desirable to be able to employ catalysts which are less problematic to handle, transport and store. Furthermore, there exists a need for polymerization processes in water, water being one of the easiest compounds to obtain access to and being a preferred solvent in numerous applications (coatings or adhesives, for example).
Nickel catalysts have been disclosed for operating in essentially organic media, as in the following documents: U.S. Pat. No. 4,711,969, U.S. Pat. No. 5,030,606 and BG 60319.
The process according to the invention involves a catalyst comprising at least one nickel or palladium or platinum atom and involves a high proportion of water.
The process according to the invention responds to the abovementioned problems and leads to a polyolefin with a high productivity in a water-rich medium. Furthermore, the process according to the invention does not require the use of a cocatalyst capable of activating the metal of the catalyst.
The invention furthermore opens a novel access route to polyolefin latices.
In the catalyst to which the invention makes recourse, the Exe2x80x94Mxe2x80x94X linkage preferably forms part of a ring comprising five atoms, two of which are carbon atoms bonded to one another via a double bond.
Generally, the metal M is bonded, before its introduction into the polymerization medium, to a ligand L. This ligand L has in particular the role of stabilizing the structure of the catalyst before it is used and of facilitating the storage and handling thereof. Before or during the polymerization, a scavenger compound is brought together with the catalyst so as to separate the ligand L from the metal M and to allow the polymerization to take place. Without the present explanation constituting in any way a limitation on the scope of the present application, it seems that the distancing of the ligand, by allowing the olefin to approach the metal M, plays an important role in the catalytic polymerization mechanism.
Thus, in the context of the present application, when a catalyst is represented so that its metal M, which can be Ni, Pd or Pt, comprises a nonattributed valency represented by a dash, as in -M, it should be understood that this valency plays the role which has just been explained, namely to be occupied by a ligand L and to be released from the ligand for the polymerization.
The catalyst may comprise only a single atom of metal M. Such a catalyst, said to be monometallic, can comprise, for example, the structure represented by the formula (1) 
in which the R1, R2, R3, R4 and R5 radicals, which can be identical or different, can be chosen from hydrogen, alkyl, cycloalkyl, aryl, alkylaryl or arylalkyl radicals, halogens, the hydroxyl radical, alkoxide radicals, xe2x80x94C(O)ORxe2x80x2, in which Rxe2x80x2 represents a hydro-carbonaceous radical which can comprise from 1 to 15 carbon atoms, or xe2x80x94SO3Y, in which Y is chosen from Li, Na, K, NH4⊕ or NRxe2x80x34⊕, in which Rxe2x80x3 represents a hydro-carbonaceous radical which can comprise from 1 to 15 carbon atoms, E, M and X having the meanings given above and it being possible for the nonattributed valency connected to M, represented by a dash in the above formula, to be occupied by a ligand L to facilitate the use of the catalyst, as has already been explained.
The linkage(s) of the Exe2x80x94Mxe2x80x94X type of the catalyst can be such that M is a nickel atom, E is an oxygen atom and X is a phosphorus atom.
The catalyst preferably comprises at least two Exe2x80x94Mxe2x80x94X linkages.
The Exe2x80x94Mxe2x80x94X linkages are preferably separated from one another via intermediate atoms bonded to one another via covalent or coordination bonds, the minimum number of atoms between two M atoms preferably ranging from 6 to 42. The term xe2x80x9cminimum number of atoms between two M atomsxe2x80x9d is understood to mean the minimum number of atoms which is encountered in the molecule of the catalyst when moving from one of the M atoms to the other of the M atoms by following the bonds atom by atom. By way of example, if the catalyst comprises the structure: 
in which Ph represents a phenyl radical, the minimum number of atoms between the Ni atoms is 8 (which corresponds to the linkage: xe2x80x94Oxe2x80x94Cxe2x80x94Cxe2x80x94Cxe2x80x94Cxe2x80x94Cxe2x80x94Cxe2x80x94Oxe2x80x94), as it is not possible to encounter fewer than 8 atoms when moving from the first Ni to the second Ni.
The catalyst may comprise only two M atoms in its structure.
The catalyst can; for example, be one of those represented by the following formula (2): 
in which the R6, R7, R8, R9, R10, R11, R12 and R13 radicals, which can be identical or different, can be chosen from the same list of radicals as R1 to R5 above, Exe2x80x2xe2x80x94Mxe2x80x2xe2x80x94Xxe2x80x2 and Exe2x80x3xe2x80x94Mxe2x80x3xe2x80x94Xxe2x80x3 being two linkages of Exe2x80x94Mxe2x80x94X type which can be identical or different and R being a bivalent radical.
The R radical can be chosen from bivalent hydrocarbonaceous radicals comprising, for example, 2 to 38 carbon atoms, such as alkylene, alkenylene, arylene, cycloalkylene, bicycloalkylene or alkylarylene radicals. The R radical can also be a 1,1xe2x80x2-ferro-cenylene radical which can be substituted, for example by one or two monovalent radicals such as xe2x80x94C(O)ORxe2x80x2 or xe2x80x94SO3Y, R and Y having the meanings already given.
By way of examples, the catalyst can be one of those comprising the structures hereinbelow: 
in which Ph represents a phenyl radical, xe2x80x94(5,6-NBEN)xe2x80x94 represents a 5,6-bicyclo[2,2,1]hept-2-ene radical, that is to say which can be represented by: 
and xe2x80x94(1,1xe2x80x2-Fc)xe2x80x94 represents a 1,1xe2x80x2-ferrocenylene radical.
In order to achieve better preservation of the integrity and thus of the effectiveness of the catalyst during its storage before use in polymerization, it is advisable to complex the atoms of metal M with a ligand L, so as to protect said M atoms by steric hindrance. Such protection is recommended in order to minimize the risks of reduction of the M atom, which can be reflected by a fall in or a loss of activity of the catalyst.
When the catalyst is used in polymerization, it is advisable to separate the ligand L from the M atom or atoms so that the latter can play their role in the activation of the polymerization reaction. The ligand can be separated before the polymerization and may even not be introduced into the polymerization medium. However, it may be left in the polymerization medium and may be even be introduced into the polymerization medium in the form complexed with the catalyst, provided that said medium comprises a scavenger compound capable of complexing of or combining in any appropriate fashion with the ligand so as to release the atoms of metal M from their complexing and thus to facilitate the polymerization. The scavenger compound must form a bond with the ligand which is sufficiently strong for the ligand to release the catalyst. It is generally possible to make use of the ligand from the synthesis of the catalyst, so that the formation of the catalyst takes place in the form complexed with the ligand.
Thus, for example, in the case of the bimetallic catalyst corresponding to the formula 
in which the R and R6 to R13 radicals have the meanings already given in the context of the formula (2), said catalysts can be prepared by reaction of a bis(xcex1-ketoylide) with a nickel(0) compound in the presence of triphenylphosphine (PPh3), which acts as ligand, according to the following reaction scheme: 
COD representing a cis,cis-1,5-cyclooctadiene radical and Ph representing a phenyl radical.
The catalyst is thus obtained by this synthesis in the complexed form, each catalyst molecule being complexed by two triphenylphosphine molecules.
In the broader context of the present application, use may generally be made, as ligand, of a compound chosen from phosphines of formula PR14R15R16, in which R14, R15 and R16, which can be identical or different, can represent alkyl, aryl, alkylaryl or arylalkyl radicals, or from phosphine oxides, ethers, esters, nitriles, ketones, amines, pyridine, substituted pyridines or alcohols.
The scavenger compound can, for example, be chosen from amine oxides, organic hydroperoxides, hydrogen peroxide or complexes of metals from Columns 8, 9 and 10 of the Periodic Table of the Elements according to the new notation as defined in the Handbook of Chemistry and Physics, 75th edition, 1994-1995, such as, for example, bis(1,5-cyclooctadiene)-nickel(0), (tetrakis(ethylene)-xcexc-dichlorodirhodium, bis(ethylene)acetylacetonerhodium(I), bis(aceto-nitrile)palladium(II), tetracarbonylnickel or triethylenenickel.
If the catalyst is introduced into the polymerization medium in the form complexed with a ligand, the scavenger compound must be introduced into the polymerization medium in an amount sufficient to release the catalyst from the ligand. Generally, the scavenger compound can be introduced into the polymerization medium in a proportion of 0.1 to 100 mol per mole of metal M contributed by the catalyst.
If the polymerization medium comprises only the aqueous phase as liquid phase, it is preferable for the catalyst and the optional scavenger compound to be soluble in said aqueous phase. In this case, it is possible to introduce, into the aqueous phase, a third organic substance which promotes the dissolution of the catalyst and/or of the optional scavenger compound in said aqueous phase for the case where this will be necessary so as to obtain complete dissolution of the catalyst and/or of the optional scavenger compound in the aqueous phase.
Such a third organic substance is thus soluble in the aqueous phase and can generally be an alcohol, such as methanol, or a ketone, such as acetone. Such a third substance can be introduced into the aqueous phase in a proportion, for example, of 5 to 15% by weight.
If the catalyst cannot be completely dissolved in the aqueous phase, it is possible to add, to the medium, a liquid organic phase which dissolves the catalyst enough for the latter to be completely dissolved in the polymerization medium, if appropriate partially in the aqueous phase and partially in the liquid organic phase.
Generally, the catalyst is completely dissolved in a liquid organic phase before the polymerization.
It is possible to introduce a liquid organic phase into the polymerization medium even if the catalyst is able to be completely dissolved in the aqueous phase.
The liquid organic phase can comprise an organic solvent and/or an olefin to be polymerized.
During the polymerization, the polymerization medium comprises the liquid aqueous phase and a solid phase composed of the solid polymer resulting from the polymerization and also comprises, according to the physical state of the olefin to be polymerized, at least one other gas phase and/or one other liquid phase. If an olefin to be polymerized is liquid under the temperature and pressure conditions of the polymerization, this olefin can form part of a liquid organic phase separate from the liquid aqueous phase. Such a liquid organic phase can also comprise an organic solvent for said olefin.
The constituents of the optional liquid organic phase are sufficiently insoluble in water for, in view of its amount involved, the aqueous phase always to comprise more than 30% of water.
The organic solvent can be chosen from saturated aliphatic, saturated alicyclic or aromatic hydrocarbons, such as, for example, isobutane, butane, pentane, hexane, heptane, isododecane, cyclohexane, cycloheptane, methylcyclohexane, methylcycloheptane, toluene, ortho-xylene or para-xylene.
The organic solvent can also be chosen from alcohols and can be a monoalcohol or a diol comprising, for example, 5 to 20 carbon atoms.
The organic solvent can be an ether comprising, for example, 3 to 15 carbon atoms, such as, for example, tetrahydrofuran or dioxane.
The organic solvent can be an ester comprising, for example, from 2 to 15 carbon atoms, such as, for example, ethyl or butyl or vinyl acetate or methyl acrylate.
For the case where the polymerization medium comprises two separate liquid phases, these can, for example, be present so that the phase other than the aqueous phase represents 1 to 50% of the volume of the aqueous phase.
The aqueous phase can comprise at least 40%, indeed even at least 50%, indeed even at least 60%, indeed even at least 70%, indeed even at least 80%, by weight of water.
The aqueous phase can comprise, in the dissolved form, an organic compound which can be an alcohol or a ketone or a diol, such as a glycol, for example ethylene glycol, or such as propanediol or butanediol. This organic compound can have the role of increasing the solubility in the aqueous phase of the olefin to be polymerized.
The catalyst is generally dissolved in at least one liquid phase before polymerization in a proportion of 0.1 micromol to 2 mol per liter and preferably of 1 micromol to 0.1 mol per liter.
Preferably, the optional scavenger compound is at least partially, and more preferably completely, dissolved in one or more liquid phases of the polymerization medium. In the case of the presence of at least one liquid organic phase, the scavenger compound will be, according to its nature and thus its affinity for one or other of the liquid phases, predominantly dissolved in the aqueous phase or predominantly dissolved in the liquid organic phase.
The polymerization medium is preferably stirred. The stirring is preferably sufficient to distribute the various phases uniformly in the reactor.
At least one dispersing agent can be added to the polymerization medium. Such a dispersing agent can in particular be used when the polymerization medium comprises a liquid organic phase, in which case it helps in the dispersion of said liquid organic phase in the form of droplets surrounded by the continuous aqueous phase. In this case, and in the case where the catalyst has been mainly dissolved in the liquid organic phase, the polymerization will mainly take place in the droplets, the latter generally exhibiting a mean diameter of between 100 xcexcm and 3 millimeters. Such a process is similar to the process referred to as xe2x80x9cradical suspension polymerizationxe2x80x9d, except that it does involve radicals.
The dispersing agent can be one of those known for having this function, such as, for example, a polyvinyl alcohol, methycellulose, a gelatin, kaolin, bariumsulfate, hydroxyapatite, magnesium silicate, tricalcium phosphate or a combination of several of these dispersing agents.
The dispersing agent can be introduced into the polymerization medium up to 10% by weight with respect to the weight of water used and preferably of 0.01% to 5% by weight with respect to the weight of water used.
At least one emulsifying agent can be added to the polymerization medium. The use of such an emulsifying agent is particularly recommended when it is desired for the polymerization to result in a latex, that is to say in an assembly of polymer particles exhibiting a mean diameter by number of less than 1 micrometer, said particles being dispersed in the aqueous phase. When an emulsifying agent is used, it is generally unnecessary for the polymerization medium to comprise a dispersing agent.
Use may be made, as emulsifying agent, of any one of the known surface-active agents, whether anionic, nonionic or even cationic. In particular, the emulsifying agent can be chosen from anionic surface-active agents, such as the sodium or potassium salts of fatty acids, in particular sodium laurate, sodium stearate, sodium palmitate or sodium oleate, the mixed sulfates of sodium or of potassium and of fatty alcohol, in particular sodium lauryl sulfate, the sodium or potassium salts of sulfosuccinic esters, the sodium or potassium salts of alkylarylsulfonic acids, in particular sodium dodecylbenzenesulfonate, and the sodium or potassium salts of monosulfonates of fatty monoglycerides, or from nonionic surfactants, such as the reaction products of ethylene oxide with alkylphenols. Mixtures of such surface-active agents can, of course, be used.
The emulsifying agent can be introduced into the polymerization medium up to 10% by weight with respect to the weight of water and preferably of 0.01% to 5% by weight with respect to the weight of water, for example of 0.01% to 3% by weight with respect to the weight of water.
In such a process comprising an emulsifying agent and in the case where it comprises a liquid organic phase, the catalyst having been mainly dissolved in said liquid organic phase and the emulsifying agent being in an amount greater than the critical micelle concentration, the polymerization takes place in the droplets of liquid organic phase, which droplets generally exhibit a mean diameter of between 1 xcexcm and 1000 xcexcm, and in the micelles, which micelles generally exhibit a mean diameter of between 1 nanometer and 100 nanometers. Such a process is similar to the process referred to as xe2x80x9cradical emulsion polymerizationxe2x80x9d, except that it does not involve radicals. When, in such a process, the concentration of emulsifying agent is increased, the relative importance of the polymerization which takes place in the micelles is increased and the formation of a latex at the end of the polymerization is promoted. In the case of the presence of a liquid organic phase, when the amount of emulsifying agent is such that all the liquid organic phase is present in the micelles, the process is similar to the process referred to as xe2x80x9cradical microemulsion polymerizationxe2x80x9d, except that the polymerization does not involve radicals.
For the case where the polymerization medium comprises a liquid organic phase and an emulsifying agent, it is possible to add a cosurfactant to the medium, as is done for miniemulsion polymerization processes. Such a cosurfactant generally exhibits a solubility in water of less than 1xc3x9710xe2x88x923 mol per liter at 20xc2x0 C. Such a cosurfactant can, for example, be hexadecane or cethyl alcohol. It can be present up to 10% by weight with respect to the weight of water and preferably the ratio of the mass of emulsifying agent to that of cosurfactant ranges from 0.5 to 2. The presence of this cosurfactant makes it possible, by virtue also of sufficient shearing of the medium, to obtain droplets of liquid organic phase of less than 1 xcexcm and promotes the formation of a latex at the end of the polymerization. Sufficient shearing can be obtained, for example, by ultrasound or by a homogenizer (such as a device of the Ultra-Turrax type or Diax 600 type from Heidolph), ultrasound being preferred. Once the characteristic size ( less than 1 xcexcm) of the droplets is obtained, stirring can be continued with less vigorous shearing, of the type of the shearing used for suspension polymerization processes.
In the case where an organic solvent has been used, the latter can, if desired, be removed by evaporation.
The process according to the invention results in polymer particles with a diameter which can range from 10 nanometers to 5 millimeters.
In the case where the polymerization comprises an emulsifying agent, a latex is obtained. On conclusion of the polymerization carried out in the presence of an emulsifying agent, the latex may comprise particles which have a tendency to separate by settling and it may be desired to carry out a separation, for example by filtration, so as to remove these particles, which do not form part of the latex.
The polymerization conditions, namely amount of the ingredients in the polymerization medium and degree of conversion of monomer to polymer, can be adjusted so that the latex exhibits a solids content ranging from 0.1 to 50% by weight.
The olefin intended to be polymerized is introduced with sufficient stirring of the polymerization medium, for example stirring ranging from 10 to 10 000 revolutions per minute. The olefin can be introduced in the liquid or gaseous form, according to its physical state.
The polymerization can take place between 0 and 300xc2x0 C. and preferably between 25 and 200xc2x0 C., at a total absolute pressure ranging from 1 to 200 bar and preferably from 1 to 100 bar. If the polymer to be formed is at least partially crystalline, the polymerization is generally carried out at a temperature below the melting point of the polymer to be formed.
In the case where only ethylene is polymerized, a high-density homopolyethylene is obtained. The polymerization of ethylene with at least one other olefin than ethylene leads to the production of an ethylene polymer with a lower density than the high-density homopolyethylene mentioned above. Depending upon the amount and the nature of the comonomer(s) of the ethylene, it is thus possible to obtain a high-density ethylene polymer (high-density polyethylene) or a medium-density ethylene polymer (medium-density polyethylene), indeed even, at a high content of comonomer, a low-density ethylene polymer (low-density polyethylene).
As is the practice for ethylene polymers, the term xe2x80x9chigh-densityxe2x80x9d is understood to mean the fact that the relative density is greater than 0.940, the term xe2x80x9cmedium-densityxe2x80x9d is understood to mean the fact that the relative density ranges from 0.925 to 0.940 and the term xe2x80x9clow-densityxe2x80x9d is understood to mean the fact that the relative density is less than 0.925.
The polymerization can thus result in a latex of a polymer of at least one olefin, that is to say in a polymer comprising polymerized units of at least one olefin, if appropriate with other units of polymerized monomer. In particular, if at least one olefin is ethylene, a latex of an ethylene polymer can be obtained.
The process according to the invention can thus result in a latex of a high-density ethylene polymer or in a latex of a medium-density ethylene polymer, indeed even of a low-density ethylene polymer.
In the context of the present application, the term xe2x80x9cpolymerxe2x80x9d must be taken in its general sense, so that it covers homopolymers, copolymers, interpolymers and blends of polymers. The term xe2x80x9cpolymerizationxe2x80x9d must also be taken in an equivalent general sense.
The group of the olefins comprises that of the xcex1-olefins. Mention may be made, as olefins, of ethylene, propylene, cyclopentene, 1-hexene, 3-methyl-1-pentene, 4-methyl-1-pentene, 1,4-hexadiene, 1,9-decadiene, 1-octene or 1-decene. The group of the olefins also includes the compounds of formula CH2xe2x95x90CHxe2x80x94(CH2)nxe2x80x94G in which n represents an integer ranging from 2 to 20 and G represents a radical which can be chosen from the following list: xe2x80x94OH, xe2x80x94CHOHCH2OH, xe2x80x94OT, xe2x80x94CF3, xe2x80x94COOT, xe2x80x94COOH, xe2x80x94Si(OH)3 or xe2x80x94Si(OT)3, T representing a hydrocarbonaceous radical comprising from 1 to 20 carbon atoms.
The process according to the invention can be carried out batchwise, semicontinuously or continuously.